This invention relates generally to sound monitoring systems and more particularly to shipborne apparatus and methods for estimating the far-field acoustics of a vessel.
Modern military ships use passive sonar techniques to detect and track target vessels. Passive sonar depends upon the detection and analysis of sound, usually machinery noise, which emanates from a target vessel and which propagates through the water to a listening vessel.
Discrete acoustic frequencies within an emanated sound, referred to as narrowband tonals, are more easily detectable at longer distances from a target vessel than the broadband frequencies radiated from the vessel. Such narrowband tonals can be directional, i.e., the amplitude of a narrowband tonal is typically a function of the radiating ship's azimuth.
The amplitude versus frequency distribution of a ship's radiated noise is commonly referred to as its acoustic signature. Military ships can detect one or more narrowband tonals of a target vessel's acoustic signature at a relatively long range from the vessel, and can analyze the tonals to track the vessel.
Understandably, it is desirable for a potential target vessel to determine its own acoustic signature to reduce its likelihood of being detected by an adversary. Understanding these directional characteristics, for example, can aid the potential target vessel by indicating what azimuthal aspect the vessel should present toward areas where listening vessels are expected to be present. Also, a knowledge of this acoustic signature can provide insight into which machinery onboard the vessel is the source of the radiated tonals, enabling the vessel to take steps to reduce these sounds.
Moreover, knowledge of one's vessel's acoustic signature can enable an analysis of this radiated noise to determine which, if any, onboard machinery may require maintenance. Machinery that has, for example, worn bearings will tend to radiate more noise than otherwise. Thus, an increase or other change in a vessel's acoustic signature may indicate the impending failure of onboard machinery, and thus alert the vessel's personnel to perform maintenance or other work on the machinery.
Accordingly, systems have been introduced for determining the acoustic signature of vessels. Under these systems, it is typical that a vessel's acoustic signature must be acquired in the Fraunhofer region of the radiated sound. This region is commonly referred to as the far-field and is defined as the distance beyond which the angular distribution of radiated energy is essentially independent of distance from the source. These systems can generally be categorized as "direct" measuring systems.
U.S. Pat. No. 5,047,990 issued to Gafos et al. discloses such a direct measuring system. While apparently effective for its intended use, the Gafos et al. system suffers from the typical drawbacks of other systems designed to measure the directionality of a vessel's noise directly, namely, the need for relatively expensive equipment and/or time-consuming monitoring procedures.